1. Field of the Invention
The present invention relates to a method of detecting a thermal asperity of a magnetic storage device for reading data from a magnetic storage medium by a magnetic resistance element and to a circuit thereof.
2. Description of the Related Art
In a magnetic disk device, a magnetic head reads and writes data from and to a rotating magnetic disk. This magnetic disk floats with rotations of the magnetic disk. For enabling a storage density of the magnetic disk device to increase, a magnetic resistance element is used as a reading element of the magnetic head.
The magnetic resistance element (MR element) is an element of which a resistance value changes corresponding to a magnitude of a magnetic field. This magnetic head reads, keeping the head itself afloat above the magnetic disk, the data from the magnetic disk. An enhancement of the storage density requires reducing a leakage of the magnetic field from the magnetic disk. It is therefore required that a floating quantity of the magnetic head be small, and this floating quantity is decreased down to several tens of microns.
On the other hand, the surface of the magnetic disk has ruggedness on the order of several microns to several tens of microns, and is not therefore perfectly flat. The magnetic head might come into contact with the rugged portion on the magnetic disk. The magnetic resistance element generates heat due to this contact. This contact heat might cause a change in resistance value of the magnetic resistance element, resulting in a change in baseline for a reading output. This is called a thermal asperity (TA) Therefore, the signal can not be normally read.
This being the case, the magnetic resistance element detects the contact with the magnetic disk, and there must be a necessity for taking some measure for the read signal, which involves a technology of detecting the thermal asperity.
FIG. 12 is a diagram showing a configuration of a prior art read circuit. FIG. 13 is a diagram showing a configuration of a prior art thermal asperity (TA) detecting circuit. FIG. 14 is a diagram showing a waveform in the prior art.
As illustrated in FIG. 12, magnetic heads 91-1, 91-2 read and write the data from and to a rotating magnetic disk 90. The two magnetic heads 91-1, 91-2 are provided for one magnetic disk 90. The magnetic heads 91-1, 91-2 has the magnetic resistance elements serving as the reading elements.
One read circuit 98 is provided for whole magnetic heads (the magnetic resistance elements) of the magnetic disk device. A switch 97 connects a selected magnetic head to one read circuit 98. A read amplifier 92 amplifies the read signals of the magnetic heads (the magnetic resistance elements) 91-1, 91-2.
A thermal asperity detecting circuit 93 detects a thermal asperity from read outputs of the magnetic heads 91-1, 91-2. A high-pass filter 94 cuts off low-frequency components of the read signals amplified by the read amplifier 92. An analog/digital converter 95 converts the read signal into a digital value. A read channel circuit 96 is constructed of a microprocessor. The read channel circuit 96 modulates the digital-valued read signals, and outputs the read data.
This conventional thermal asperity detecting circuit 93 is, as illustrated in FIG. 13, constructed of an electric potential source 100 for generating a slice level SL, and a comparing circuit 99 for comparing levels of the read signal of the magnetic heads (the magnetic resistance elements) 91-1, 91-2 with the slice level SL.
As shown in FIG. 14, a read output RS-1 of the magnetic resistance element when not in contact with the magnetic disk 90 is fixed in terms of a baseline. When the magnetic resistance element is brought into contact with the magnetic disk 90, however, the read output changes as indicated by RS-2 in FIG. 14. Namely, when the magnetic resistance element comes into contact with the magnetic disk 90, the baseline of the read output RS-2 of the magnetic resistance element abruptly rises due to the heat of the magnetic resistance element. Then, the baseline gradually returns to the previous value with a thermal decrease of the magnetic resistance element.
For detecting the thermal asperity indicating the output change described above, the read signal has hitherto been compared with the fixed slice level SL common to the respective heads. Then, if the read level exceeds the slice level SL, a thermal asperity detection signal TAF is generated.
The read channel circuit 96 is notified of this thermal asperity detection signal TAF, and executes a process of correcting the baseline of the read signal. For example, the read channel circuit 96, upon receiving the thermal asperity detection signal TAF, retries to read the data on the track concerned.
Then, the read channel circuit 96, when in the read-retry process, operates the high-pass filter 94 and cuts off the low-frequency component of the read signal RS-2. A frequency of change in the baseline is lower than a frequency of a data part of the read signal. Therefore, the read signal RS-2 is processed so that its low-frequency component of the segment with the changed baseline is cut off as indicated by a waveform RS-3 in FIG. 14. A time (a length) of the segment with the changed baseline is thereby reduced.
This segment with the changed baseline is small, and therefore, even if the data of this segment is judged to be abnormal, this can be saved by an error correction which will be executed at a posterior stage. Incidentally, the reason why the high-pass filter is operated only when detecting the thermal asperity is that the low-frequency component of the read signal is always cut off if the high-pass filter is operated at all times, and the signal level is lowered, resulting in causing more reading errors. The high-pass filter is operated only when the thermal asperity is detected, thereby making it feasible to restrain the descent of the signal level within a minimum range.
Thus, the slice level has hitherto been set based on the absolute value common to the respective heads.
There arise, however, the following problems inherent in the prior art.
First, the single read circuit 98 (the thermal asperity detecting circuit 93) is provided for the whole respective heads of the magnetic disk device. The output level of each of the magnetic heads (the magnetic resistance elements) differs corresponding to differences in terms of the resistance values of the magnetic resistance elements and of sense currents. The prior art has such a problem that the thermal asperity of each magnetic resistance element is unable to be accurately detected because of the common slice level being set with respect to the output level differing according to each magnetic resistance element.
Second, avoidance of this problem involves the use of a method of measuring the output level of each of the magnetic resistance elements and setting the slice level for detecting the thermal asperity of each individual magnetic resistance element. This method presents an inconvenience, wherein a characteristic of each of the magnetic resistance elements within the magnetic disk device is measured, and the slice level must be determined. A further inconvenience peculiar to this method is that a memory is stored with the slice levels of the respective magnetic resistance elements, and, when selecting the magnetic head (the magnetic resistance element), the slice level of the selected magnetic head (the magnetic resistance element) is required to be set in the thermal asperity detecting circuit. This conduces to such a problem that a measuring process is time-consuming due to the detection of the thermal asperity, and the control is needed when selecting the head.